The 6 April 2009, Mw 6.3, L’Aquila (Central Italy) earthquake: strong-motion observations

On 6 April 2009, 01:32:40 UTC, an Mw 6.3 earthquake occurred in the Abruzzo region (central Italy), close to L’Aquila, a town of 68,500 inhabitants. About 300 people died because of the collapse of many residential and public build¬ings, and damage was widespread in L’Aquila and its neighbor¬ing municipalities.
The earthquake occurred at 9.5 km depth along a NW-SW normal fault with SW dip, located below the city of L’Aquila (Istituto Nazionale di Geofisica e Vulcanologia [INGV] 2009a). The maximum observed intensity is IX–X in the MCS scale and the most relevant damages are distributed in the NW-SE direc¬tion, with evident predominance toward the southeast (Istituto Nazionale di Geofisica e Vulcanologia 2009b). This event rep¬resents the third largest earthquake recorded by strong-motion instruments in Italy, after the 1980 Mw 6.9 Irpinia and the 1976 Mw 6.4 Friuli earthquakes (Luzi et al. 2008).
The mainshock was followed by seven aftershocks of moment magnitude larger than or equal to 5, the two stron¬gest of which occurred on April 7 (Mw = 5.6) and April 9 (Mw = 5.4). The mainshock and its aftershocks have been recorded by several digital stations of the Italian strong-motion network (Rete Accelerometrica Nazionale, R AN), operated by the Italian Department of Civil Protection (DPC); by the Italian seismometric network (Rete Sismometrica Nazionale, operated by INGV-Centro Nazionale Terremoti (CNT); http://cnt. rm.ingv.it); and by a temporary strong-motion array installed by the INGV Sezione di Milano-Pavia (MI-PV; http://www. mi.ingv.it).
A total of 56 three-component strong-motion record¬ings were obtained within 280 km for the mainshock, with 23 being within 100 km of the epicenter. Horizontal peak ground motions in the near-fault region range from 327 to 646 cm/sec2, the latter representing one of the highest values recorded in Italy. This strong-motion data set, consisting of 954 waveforms from Mw > 4.0 events, is unique in Italy because it is entirely digital and includes observations from near-fault dis¬tances to some hundred kilometers away. The data set has been integrated in the new Italian strong-motion database ITACA (ITalian ACcelerometric Archive), available at http://itaca. mi.ingv.it.
This paper provides an overview of the strong-motion recordings of the mainshock and the two strongest aftershocks with preliminary analyses of different strong-motion param¬eters as a function of distance, azimuth, and site conditions.